The majority of suspended or “dropped” ceiling construction in use today employs so-called T-bar rails, having the cross-section of an inverted “T”, arranged in a rectilinear grid and suspended from the structural ceiling by tie wires or metal straps. The system is essentially that described in U.S. Pat. No. 2,710,679 (granted to Bibb et al. on Jun. 14, 1955), with minor modernizations. Rectangular ceiling tiles, generally either porous acoustic tile or decorative panels, are inserted between the T-bars at an angle, leveled, and dropped into the grid, where they rest on the horizontal flanges of the inverted “T”. In the United States, the cell size in the suspension grid is typically either 24 in×24 in or 24 in×48 in, while in Europe and elsewhere the cell size in the suspension grids is 600 mm×600 mm (23.62 in×23.62 in) or 600 mm×1200 mm (23.62 in×47.24 in). The ceiling tiles are actually about 6 mm ( 1/4  inch) smaller than the nominal (i.e., cell) size, to facilitate easy installation between the vertical stems of the T-bars.
The popularity of this construction is due to the ease and low cost of installation, and to the fact that the individual tiles are readily pushed up and off of the rails whenever access to the space above the ceiling is required, and can be returned to their original placement without damage. Tiles having any desired finish and appearance can be manufactured to fit into the standard T-bar grids, giving decorators and architects a wide range of design choices. Lighting fixtures and air diffusers and grilles, built to the same dimensions as the tiles, can be dropped into the grid wherever desired. Tiles and fixtures of the standard dimensions are commercially available from a wide range of sources.
One disadvantage of this system is that, in an earthquake, the tiles and fixtures can bounce up and off of the T-bar flanges, and then drop to the floor or onto the building's occupants, as a consequence of not being mechanically connected or attached to the T-bars. As tiles fall from their places, the suspended grid becomes flexible and prone to even greater movement and distortion, causing more tiles to fall; the result is often a progressive failure of the entire ceiling.
In earthquake-prone areas, seismic building codes often require splayed (diagonal) tie wires to be installed, to limit lateral motion and distortion of the grid during an earthquake. Vertical posts are sometimes installed as well, to limit vertical motion of the grid. Such preventive measures render the grid more rigid, and ensure that it moves along with (and not relative to) the building, but they add to the labor and expense of installation, and they do not entirely prevent individual tiles and fixtures from separating from the T-bars. Fixing the tiles to the T-bars, for example by installation of retention clips, is labor-intensive, and interferes with easy access to the space above the ceiling. Easily accessed clips tend to be visible, and can mar the aesthetics of the ceiling design. Safety mechanisms that “catch” falling tiles (e.g., U.S. Pat. No. 5,253,463 granted to Witmyer on Oct. 19, 1993) still permit the tiles to separate from the T-bars, and the grid can still suffer from the resulting loss of rigidity.
Tiles having a slot or kerf along the sides, into which the T-bar horizontal flanges are fitted, are known. Kerfed tiles are intended to conceal the grid, partially or completely, from view from below, and by virtue of being locked to the grid, they also have improved seismic resistance. Tiles having four kerfed sides are rarely employed, because they must be slid into place as the T-bar grid is being assembled, and they present an installation problem when the assembly process reaches a wall. There are kerfed tiles designed for installation in a pre-existing grid, which feature some combination of breaks in the flanges and/or the upper lips of the kerfs, that permit the tiles to be slid into place. Tiles featuring a small upper lip along two adjacent kerfs, that take advantage of the ¼-inch of leeway between tile and grid to enable installation, are known, but such tiles are not truly locked to the grid. Kerfed tiles having gaskets, that snap into place over and below the T-bar horizontals, are known (e.g., U.S. Pat. No. 4,760,677 granted to Nassof on Aug. 2, 1988, and U.S. Pat. No. 5,507,125 granted to McClure on Apr. 16, 1996). Removal of kerfed tiles without damage, for access to the space above the ceiling, can be difficult or impossible, particularly when the method of installation is not apparent to the person attempting the removal.
Separate frames intended to obscure the T-bar are known (e.g., U.S. Pat. No. 4,980,957 granted to Bumpus et al. on Jan. 1, 1985), but these frames, which serve only an aesthetic purpose, clip to the T-bar and do not secure the ceiling tile. There is a need for a suspended ceiling system that remains easy to install and maintain, but which does not drop tiles in the event of an earthquake. Similar needs exist in mobile environments, such as military and passenger ships, where ceiling structures are sometimes subjected to unusual forces and motions.
A feature of suspended ceilings is the air space, or plenum, between the suspended tiles and the structural ceiling above. If ductwork for both a forced-air supply and forced-air return is installed, the airspace is “dead”, i.e., filled with non-circulating air. In the absence of return air ducts, the plenum is usually provided with an exit duct, and the space above the tiles is an “active” plenum filled with circulating air. Electrical wiring installed in an active plenum can represent a fire hazard, because toxic gases and smoke from burning insulation and plastics are not contained, as they would be in a dead airspace, but are passed directly into the building's air circulation system. Another hazard is that a fire in a plenum space could spread rapidly before being detected, if combustible materials are present.
When the airspace above a dropped ceiling is used as an active plenum, construction standards and/or local fire regulations require low-voltage cables and wiring either to be installed inside metal conduit, or else provided with low-smoke/low-toxicity wire insulation which does not support combustion on its own. Twisted pair and coaxial cables, for telephone and data network services, are the most common form of wiring found above ceilings in commercial buildings. Specialized plenum (or plenum-rated) cable is referred to as Low Smoke Zero Halogen (LSZH or LSOH) cable. Plenum-rated cable is generally insulated and sheathed with fluorocarbon polymers, which makes it significantly more costly than equivalent non-plenum-rated wiring, which typically has inexpensive polyethylene insulation and PVC sheathing.
High-voltage electrical equipment and wiring (generally, >50 volts) above a ceiling is required to be enclosed in metal conduit or raceways, and must be physically isolated from low-voltage wiring. Devices and fixtures, such as lighting fixtures, must be enclosed in metallic boxes. Electrical outlets are permitted inside the plenum space (if enclosed within electrical boxes), but because the sockets themselves must be located on the exterior of the dropped ceiling, plug-in connection of fixtures is impractical. The overall result is that all fixtures and devices installed in a ceiling must be hard-wired, using metal conduits and junction boxes.
Meeting these construction and fire codes adds substantially to the time and cost of installation, as the conduit and boxes represent added capital costs, and require a considerable amount of skilled labor to install. It is particularly difficult and costly to add high-voltage wiring to a previously installed system. There is a need for suspended ceilings that can safely be wired without the added expense of conduit, junction boxes, and plenum-rated wiring, and which permit the plug-in connection of electrical fixtures.